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Microbiological problems and biofilms associated with Mycobacterium chimaera in heater–cooler units used for cardiopulmonary bypass

  • J. Walker
    Correspondence
    Corresponding author. Address: Biosafety, Air and Water Microbiology Group, Public Health England, National Infection Service, Porton Down, Salisbury SP4 0JG, UK. Tel.: +44 (0)1980 612643.
    Affiliations
    Biosafety, Air and Water Microbiology Group, National Infection Service, Public Health England, Porton Down, Salisbury, UK
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  • G. Moore
    Affiliations
    Biosafety, Air and Water Microbiology Group, National Infection Service, Public Health England, Porton Down, Salisbury, UK
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  • S. Collins
    Affiliations
    Biosafety, Air and Water Microbiology Group, National Infection Service, Public Health England, Porton Down, Salisbury, UK
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  • S. Parks
    Affiliations
    Biosafety, Air and Water Microbiology Group, National Infection Service, Public Health England, Porton Down, Salisbury, UK
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  • M.I. Garvey
    Affiliations
    Infection Prevention and Control Team, University Hospitals Birmingham NHS Foundation Trust, Queen Elizabeth Hospital Birmingham, Mindelsohn Way, Edgbaston, Birmingham, UK
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  • T. Lamagni
    Affiliations
    Healthcare-Associated Infection & Antimicrobial Resistance Department, National Infection Service, Public Health England, Colindale, London, UK
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  • G. Smith
    Affiliations
    Public Health England National Mycobacterial Reference Service, Birmingham Public Health Laboratory, Birmingham, UK
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  • L. Dawkin
    Affiliations
    Estates and Facilities, University Hospitals Coventry and Warwickshire NHS Trust, Walsall, UK
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  • S. Goldenberg
    Affiliations
    Centre for Clinical Infection and Diagnostics Research, King’s College, London and Guy’s & St Thomas’ NHS Foundation Trust, London, UK
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  • M. Chand
    Affiliations
    Reference Microbiology, National Infection Service, Public Health England, Colindale, London, UK

    Guy’s & St Thomas’ NHS Foundation Trust, London, UK

    National Institute for Health Research, Health Protection Research Unit in Respiratory Infections, Imperial College, London, UK
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Published:April 26, 2017DOI:https://doi.org/10.1016/j.jhin.2017.04.014

      Summary

      The role of heater–cooler units (HCUs) in the transmission of Mycobacterium chimaera during open heart surgery has been recognized since 2013. Subsequent investigations uncovered a remarkable global outbreak reflecting the wide distribution of implicated devices. HCUs are an essential component of cardiopulmonary bypass operations and their withdrawal would severely affect capacity for life-saving cardiac surgery. However, studies have demonstrated that many HCUs are contaminated with a wide range of micro-organisms, including M. chimaera and complex biofilms. Whole genome sequencing of M. chimaera isolates recovered from one manufacturer’s HCUs, worldwide, has demonstrated a high level of genetic similarity, for which the most plausible hypothesis is a point source contamination of the devices. Dissemination of bioaerosols through breaches in the HCU water tanks is the most likely route of transmission and airborne bacteria have been shown to have reached the surgical field even with the use of ultraclean theatre ventilation. Controlling the microbiological quality of the water circulating in HCUs and reducing biofilm formation has been a major challenge for many hospitals. However, enhanced decontamination strategies have been recommended by manufacturers, and, although they are not always effective in eradicating M. chimaera from HCUs, UK hospitals have not reported any new cases of M. chimaera infection since implementing these mitigation strategies. Water safety groups in hospitals should be aware that water in medical devices such as HCUs may act as a vector in the transmission of potentially fatal water-borne infections.

      Keywords

      Introduction

      Mycobacterium chimaera infections following cardiac surgery have been attributed to the use of heater–cooler units (HCUs) which form part of the cardiopulmonary bypass equipment. Cases have been reported worldwide reflecting the global problem of microbial colonization and the need for effective management of HCUs [
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      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
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      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
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      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. This review summarizes the knowledge to date as well as the challenges faced by those who have to prevent and control infection.

      Burden and history of non-tuberculous mycobacteria

      Water-borne opportunistic pathogens are responsible for a range of infections in immunocompromised and vulnerable patients, especially in healthcare facilities. Organisms including Legionella spp., Pseudomonas aeruginosa and non-tuberculous mycobacteria (NTM) have been recovered from water systems and may cause persistent problems in hospitals [
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      Non-tuberculous mycobacteria as water-borne pathogens

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      ]. However, unlike legionella, the control of NTM is not covered by any specific legislation or guidance [
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      ]. Individuals tend to be predisposed to NTM infection, e.g. have comorbidities and/or are immunosuppressed, but infections are increasingly being seen in otherwise healthy individuals [
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      ].
      In the majority of countries, the most common manifestation of NTM infection is sporadic pulmonary disease caused by members of the Mycobacterium avium complex (MAC) – a group of organisms that includes M. avium, M. intracellulare and M. chimaera [
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      ].
      Members of the MAC are relatively slow-growing and can take upwards of six weeks to culture from patient and environmental samples. Delays in identification and diagnosis, coupled with the fact that MAC infections do not require notification to public health teams, mean that the true burden of these organisms within healthcare facilities in the UK is difficult to quantify.

      Non-tuberculous mycobacteria and cardiac surgery

      Exposure to water droplets aerosolized from contaminated hospital water systems has been linked to pulmonary MAC infection but other routes of infection are possible. In 1976, NTM infections following cardiac surgery were attributed to porcine heart valves contaminated with M. fortuitum and M. chelonae [
      • Falkinham 3rd, J.O.
      • Hilborn E.D.
      • Arduino M.J.
      • Pruden A.
      • Edwards M.A.
      Epidemiology and ecology of opportunistic premise plumbing pathogens: Legionella pneumophila, Mycobacterium avium, and Pseudomonas aeruginosa.
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      • Kline S.
      • Cameron S.
      • Streifel A.
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      • et al.
      Mycobacterium xenopi spinal infections after discovertebral surgery: investigation and screening of a large outbreak.
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      Mycobacterium fortuitum epidemics after open-heart surgery.
      ,
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      • O'Bar P.R.
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      Mycobacterium fortuitum epidemics after open heart surgery.
      ,
      • Robicsek F.
      • Hoffman P.C.
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      • Daugherty H.K.
      • Cook J.W.
      • Selle J.G.
      • et al.
      Rapidly growing nontuberculous mycobacteria: a new enemy of the cardiac surgeon.
      ]. Inadequate sterilization had allowed NTM to survive on the surface of porcine valves [
      • Robicsek F.
      • Hoffman P.C.
      • Masters T.N.
      • Daugherty H.K.
      • Cook J.W.
      • Selle J.G.
      • et al.
      Rapidly growing nontuberculous mycobacteria: a new enemy of the cardiac surgeon.
      ]. The valves were recalled and, following changes in procurement and sterilization, contamination rates were reduced from about 4% to <0.0001%.
      In 1992, Hector et al. described four nosocomial outbreaks of M. fortuitum. All cases followed cardiac surgery and two were attributed to the hospital potable water supplies [
      • Hector J.S.R.
      • Pang Y.
      • Mazurek G.H.
      • Zhang Y.
      • Brown B.A.
      • Wallace R.J.
      Large restriction fragment patterns of genomic Mycobacterium fortuitum DNA as strain-specific markers and their use in epidemiologic investigation of four nosocomial outbreaks.
      ]. To determine the source of M. wolinskyi bacteraemia and aortic prosthesis colonization (only 16 days after surgery), Dupont et al. sampled a range of environmental surfaces and water sources [
      • Dupont C.
      • Terru D.
      • Aguilhon S.
      • Frapier J.M.
      • Paquis M.P.
      • Morquin D.
      • et al.
      Source-case investigation of Mycobacterium wolinskyi cardiac surgical site infection.
      ]. Although M. wolinskyi was not recovered, M. chelonae, M. mucogenicum, and M. llaterense were detected in potable water sources.

      Heater–cooler units as reservoirs of non-tuberculous mycobacteria

      The first report of M. chimaera infections following cardiac surgery was published in 2013 [
      • Achermann Y.
      • Rossle M.
      • Hoffmann M.
      • Frapier J.M.
      • Paquis M.P.
      • Morquin D.
      • et al.
      Prosthetic valve endocarditis and bloodstream infection due to Mycobacterium chimaera.
      ]. A follow-up investigation by Sax et al. described a total of six M. chimaera prosthetic valve endocarditis or vascular graft infections and an epidemiological link to HCUs used during surgery [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ]. Since then, additional reports of M. chimaera infections associated with the use of HCUs have emerged from Australia, Canada, France, Germany, Hong Kong, Ireland, Netherlands, Spain, Switzerland, UK, and USA, reflecting the global use of these devices [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ,
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Department of Health
      Therapeutic Goods Administration, Australian Government. Non-tuberculous mycobacterium infections associated with heater-cooler devices. Alert – updated advice for health professionals and facilities following report of patient infection.
      ,
      • Health Canada
      Heater-cooler devices – risk of nontuberculous Mycobacteria infections.
      ,
      • European Centre for Disease Prevention and Control
      Invasive cardiovascular infection by Mycobacterium chimaera associated with 3T heater–cooler system used during open-heart surgery.
      ,
      • Kohler P.
      • Kuster S.P.
      • Bloemberg G.
      • Schulthess B.
      • Frank M.
      • Tanner F.C.
      • et al.
      Healthcare-associated prosthetic heart valve, aortic vascular graft, and disseminated Mycobacterium chimaera infections subsequent to open heart surgery.
      ,
      • Government of the Hong Kong Special Administrative Region
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      ,
      • Cai Y.
      • Landolfo K.
      • Renew J.R.
      Mycobacterium infection from a cardiopulmonary bypass heater-cooler unit in a patient with steroid-induced immunosuppression.
      ,
      • Tan N.
      • Sampath R.
      • Abu Saleh O.M.
      • Tweet M.S.
      • Jevremovic D.
      • Alniemi S.
      • et al.
      Disseminated Mycobacterium chimaera infection after cardiothoracic surgery.
      ].
      Interestingly, the potential significance of HCUs in infection control was determined as early as 2002 in a publication that highlighted HCU microbial contamination, malfunctions, possible patient exposure routes, incompatibility of biocides with HCU components and problems regarding effective decontamination [
      • Weitkemper H.H.
      • Spilker A.
      • Knobl H.J.
      • Korfer R.
      The heater–cooler unit – a conceivable source of infection.
      ]. These are all issues that are again discussed in light of recent investigations [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ].

      What is a heater–cooler and how do they operate?

      Heater–cooler units are integral for cardiopulmonary bypass operations and can be used for extracorporeal membrane oxygenation (ECMO) or extracorporeal life support [
      • Trudzinski F.
      • Schlotthauer U.
      • Kamp A.
      • Hennemann K.
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      • Reischl U.
      • et al.
      Clinical implications of Mycobacterium chimaera detection in thermoregulatory devices used for extracorporeal membrane oxygenation (ECMO), Germany, 2015 to 2016.
      ]. They have two or three interconnected cold and warm water tanks which function in combination, to deliver temperature-controlled water to external heat exchangers associated with cardioplegia and oxygenation equipment as well as the patient heating blanket (Figure 1). The temperature difference between the water circulated by the HCU and the blood determines heat transfer and regulates the temperature of the blood perfusing the patient. Heat exchangers should prevent any contact between the HCU water and the patient, although rare leaks causing infection have been described [
      • Geldof W.C.
      • Brom A.G.
      Infections through blood from heart–lung machine.
      ,
      • Gukop P.
      • Tiezzi A.
      • Mattam K.
      • Sarsam M.
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      ]. The sealed water tanks contain a number of different materials such as plastic tubing, insulation material, stainless steel and copper. Some models have two overflow pipes which can be used to vary the volume of water contained within (Figure 2). However, as only one of these overflow pipes is used at any one time, the other becomes a dead-leg, resulting in stagnation of water, debris accumulation, and formation of biofilm. The overflow bottle and the dead-leg, which are usually filled with tank water, must also be included in the decontamination regimen.
      Figure 1
      Figure 1Schematic of a typical heater–cooler unit, oxygenator and cardioplegia equipment.
      Figure 2
      Figure 2Two overflow pipes which can be used to vary the volume of water in LivaNova 3T heater–cooler unit (HCU) tanks. The lower pipe takes the overflow (and visible debris) from the tank to the overflow bottle that is attached to the rear of the HCU. The upper pipe is blocked at the top and contains stagnant water, biofilm and debris.

      Determining the source of infections related to heater–cooler units

      The water circulating in an HCU may contain a wide range of micro-organisms. Extremely high heterotrophic plate counts (>108 cfu/L) comprising a range of environmental Gram-negative organisms including P. aeruginosa, Sphingomonas paucimobilis, Stenotrophomonas maltophilia and Brevundimonas vesicularis have been reported [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ]. Other opportunistic pathogens including M. chelonae, Aspergillus flavus and non-aspergillus moulds (Paecilomyces spp.) have been identified. The presence of L. pneumophila has also been detected [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Dupont C.
      • Terru D.
      • Aguilhon S.
      • Frapier J.M.
      • Paquis M.P.
      • Morquin D.
      • et al.
      Source-case investigation of Mycobacterium wolinskyi cardiac surgical site infection.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ,
      • Diaz-Guerra T.M.
      • Mellado E.
      • Cuenca-Estrella M.
      • Gaztelurrutia L.
      • Navarro J.I.V.
      • Tudela J.L.R.
      Genetic similarity among one Aspergillus flavus strain isolated from a patient who underwent heart surgery and two environmental strains obtained from the operating room.
      ]. The aerosolization of L. pneumophila could represent a risk to users and surrounding staff [
      • Weitkemper H.H.
      • Spilker A.
      • Knobl H.J.
      • Korfer R.
      The heater–cooler unit – a conceivable source of infection.
      ]. However, in the UK, no cases of Legionnaires’ disease associated with HCUs have been identified [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. The presence of high numbers and a diverse range of micro-organisms within an HCU is likely to facilitate the formation of extensive biofilms on tubing associated with the water tanks (Figure 3) [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ].
      Figure 3
      Figure 3Presence of extensive biofilm on materials in the water tanks of the LivaNova PLC, Sorin 3T heater–cooler unit, studied at Public Health England, Porton Down, UK.
      During their outbreak investigations, Sax et al. recovered M. chimaera from water taken from five different HCUs [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ]. Similar randomly amplified polymorphic DNA polymerase chain reaction patterns were observed in isolates recovered from one of these HCUs and an associated air sample implying the aerosolization of M. chimaera from within the unit. However, no isolate recovered from any of the HCUs was an exact match to that from an infected patient, and it was hypothesized that this was due to the long time-lag between patient exposure and the sampling of the HCU [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ]. Samples taken from drinking-water fountains were also positive for M. chimaera but it was concluded that drinking water was unlikely to be responsible for the invasive infections. Chand et al. sampled 45 HCUs across 12 different UK hospitals and 19 (42%) were positive for M. chimaera. The results of these and other studies suggest that M. chimaera is a widely occurring contaminant of HCUs and that contaminated water within HCUs could be the source of M. chimaera infections [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ,
      • Perkins K.M.
      • Lawsin A.
      • Hasan N.A.
      • Strong M.
      • Halpin A.L.
      • Rodger R.R.
      • et al.
      Notes from the field: Mycobacterium chimaera contamination of heater–cooler devices used in cardiac surgery – United States.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ,
      • Haller S.
      • Holler C.
      • Jacobshagen A.
      • Hamouda O.
      • Abu Sin M.
      • Monnet D.L.
      • et al.
      Contamination during production of heater–cooler units by Mycobacterium chimaera potential cause for invasive cardiovascular infections: results of an outbreak investigation in Germany, April 2015 to February 2016.
      ].
      To date, all M. chimaera infections have been attributed to a specific make/model of HCU (Sorin 3T, LivaNova PLC, formerly Sorin Group Deutschland GmbH). M. chimaera has been recovered from in-use 3T HCUs within hospitals worldwide, from new unused 3T HCUs, and from water samples obtained at the manufacturing site of LivaNova PLC [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ,
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • European Centre for Disease Prevention and Control
      Invasive cardiovascular infection by Mycobacterium chimaera associated with 3T heater–cooler system used during open-heart surgery.
      ,
      • Kohler P.
      • Kuster S.P.
      • Bloemberg G.
      • Schulthess B.
      • Frank M.
      • Tanner F.C.
      • et al.
      Healthcare-associated prosthetic heart valve, aortic vascular graft, and disseminated Mycobacterium chimaera infections subsequent to open heart surgery.
      ,
      • Diaz-Guerra T.M.
      • Mellado E.
      • Cuenca-Estrella M.
      • Gaztelurrutia L.
      • Navarro J.I.V.
      • Tudela J.L.R.
      Genetic similarity among one Aspergillus flavus strain isolated from a patient who underwent heart surgery and two environmental strains obtained from the operating room.
      ,
      • Williamson D.
      • Howden B.
      • Stinear T.
      Mycobacterium chimaera spread from heating and cooling units in heart surgery.
      ,
      • Centers for Disease Control and Prevention
      Contaminated devices putting open-heart surgery patients at risk.
      ]. However, microbial contamination of HCUs is widespread and is not specific to particular manufacturers or models [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Svensson E.
      • Jensen E.T.
      • Rasmussen E.M.
      • Folkvardsen D.B.
      • Norman A.
      • Lillebaek T.
      Mycobacterium chimaera in heater–cooler units in Denmark related to isolates from the United States and United Kingdom.
      ]. Such contamination implies a systemic failure of the decontamination regimen resulting in the formation of biofilms. The environmental conditions and usage of the HCU favours the growth of a wide range of micro-organisms [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ,
      • Haller S.
      • Holler C.
      • Jacobshagen A.
      • Hamouda O.
      • Abu Sin M.
      • Monnet D.L.
      • et al.
      Contamination during production of heater–cooler units by Mycobacterium chimaera potential cause for invasive cardiovascular infections: results of an outbreak investigation in Germany, April 2015 to February 2016.
      ]. In operating theatres, the HCU is set up and connected to allow water to circulate through the unit, tubing, cardioplegia equipment, and oxygenator some hours prior to the entry of the patient. This gives time to ensure that all the equipment is operating appropriately but also for the temperatures to stabilize. The temperature of the water circulating in the HCU may be as high as 40°C; this temperature encourages bacterial growth and formation of biofilm [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ]. When not in use the 3T HCUs are stored at ambient temperatures allowing bacterial survival and proliferation of the biofilm in the stagnant water, particularly on pipe materials and inside the water tanks (Figure 2, Figure 3). Biofilms produce extensive extracellular polysaccharides (EPS) which present a physicochemical barrier to the penetration of disinfectants [
      • Daddi Oubekka S.
      • Briandet R.
      • Fontaine-Aupart M.P.
      • Steenkeste K.
      Correlative time-resolved fluorescence microscopy to assess antibiotic diffusion-reaction in biofilms.
      ]. As such the tolerance of biofilms is thought to be associated with the quenching of disinfectants as they pass through the EPS and due to the presence of slow-growing or persistor cells that have reduced susceptibility to many disinfectants and other antimicrobials [
      • Flemming H.C.
      • Wingender J.
      The biofilm matrix.
      ,
      • Billings N.
      • Birjiniuk A.
      • Samad T.S.
      • Doyle P.S.
      • Ribbeck K.
      Material properties of biofilms – a review of methods for understanding permeability and mechanics.
      ,
      • Brown M.R.
      • Allison D.G.
      • Gilbert P.
      Resistance of bacterial biofilms to antibiotics: a growth-rate related effect?.
      ]. Resistance to disinfectants may also be due to genetic characteristics acquired either by mutation or by gene exchange [
      • Flemming H.C.
      • Wingender J.
      • Szewzyk U.
      • Steinberg P.
      • Rice S.A.
      • Kjelleberg S.
      Biofilms: an emergent form of bacterial life.
      ].
      In terms of mycobacteria, the presence of a high cell-surface hydrophobicity may result in their preferential attachment to surfaces at air–water interfaces and may contribute to disinfectant tolerance of NTM within the HCU [
      • Van Oss C.J.
      • Gillman C.F.
      • Neumann A.W.
      Phagocytic engulfment and cell adhesivness.
      ]. Hydrophobicity varies between MAC species [
      • Stormer R.S.
      • Falkinham 3rd, J.O.
      Differences in antimicrobial susceptibility of pigmented and unpigmented colonial variants of Mycobacterium avium.
      ]. Both M. avium and M. intracellulare are recovered more frequently from pipe surfaces in biofilms than in the planktonic phase [
      • Falkinham 3rd, J.O.
      • Norton C.D.
      • LeChevallier M.W.
      Factors influencing numbers of Mycobacterium avium, Mycobacterium intracellulare, and other Mycobacteria in drinking water distribution systems.
      ,
      • Torvinen E.
      • Suomalainen S.
      • Lehtola M.J.
      • Miettinen I.T.
      • Zacheus O.
      • Paulin L.
      • et al.
      Mycobacteria in water and loose deposits of drinking water distribution systems in Finland.
      ]. However, M. intracellulare forms biofilms more readily than the less hydrophobic M. avium, which is more widely found in the bulk water [
      • Falkinham 3rd, J.O.
      • Norton C.D.
      • LeChevallier M.W.
      Factors influencing numbers of Mycobacterium avium, Mycobacterium intracellulare, and other Mycobacteria in drinking water distribution systems.
      ,
      • Kirschner Jr., R.A.
      • Parker B.C.
      • Falkinham 3rd, J.O.
      Epidemiology of infection by nontuberculous mycobacteria. Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium scrofulaceum in acid, brown-water swamps of the southeastern United States and their association with environmental variables.
      ]. The presence of NTM biofilms in HCUs makes their decontamination extremely challenging and hospitals are still reporting intermittent positives of HCUs despite complying with the recommended disinfection procedure(s) (J. Walker, personal communication).
      Despite the global distribution and use of 3T HCUs, whole genome analysis has demonstrated a lack of genetic diversity between isolates [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ,
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. In Australia, M. chimaera was recovered from 10 HCUs across four different hospitals [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ]. Representative isolates clustered into two groups which differed by 28 single nucleotide polymorphisms (SNPs), with 2–17 SNP differences between isolates within a single group [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ]. Recent evidence also suggests that a high level of DNA sequence conservation (equivalent to six SNPs) exists between isolates recovered from the southern and northern hemispheres [
      • Williamson D.
      • Howden B.
      • Stinear T.
      Mycobacterium chimaera spread from heating and cooling units in heart surgery.
      ]. During an outbreak investigation in the USA, genomic analysis identified a maximum of 38 SNPs between any two isolates in the outbreak. Of the 261 isolates examined by Chand et al., 94% (including all those from probable cases and 86% of isolates from 3T HCUs) clustered very closely (mean pairwise distance of 10 SNPs) [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. By comparison, a minimum of 2900 SNPs were detected between any single outbreak isolate and an epidemiologically unlinked isolate [
      • Perkins K.M.
      • Lawsin A.
      • Hasan N.A.
      • Strong M.
      • Halpin A.L.
      • Rodger R.R.
      • et al.
      Notes from the field: Mycobacterium chimaera contamination of heater–cooler devices used in cardiac surgery – United States.
      ]. Such data strongly indicate a point source for the contamination of HCUs, i.e. that the LivaNova 3T units were contaminated within the manufacturing site [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ,
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. Preliminary data from Haller et al. support this hypothesis [
      • Haller S.
      • Holler C.
      • Jacobshagen A.
      • Hamouda O.
      • Abu Sin M.
      • Monnet D.L.
      • et al.
      Contamination during production of heater–cooler units by Mycobacterium chimaera potential cause for invasive cardiovascular infections: results of an outbreak investigation in Germany, April 2015 to February 2016.
      ]. However, Svensson et al. provided evidence that HCUs from other manufacturers (e.g. Maquet) have cultured M. chimaera [
      • Svensson E.
      • Jensen E.T.
      • Rasmussen E.M.
      • Folkvardsen D.B.
      • Norman A.
      • Lillebaek T.
      Mycobacterium chimaera in heater–cooler units in Denmark related to isolates from the United States and United Kingdom.
      ]. However, these M. chimaera isolates were genetically distinct (47–49 SNP differences) compared with isolates from the Sorin 3T HCUs and have not been linked with any clinical cases. In addition, other manufacturer units have been shown to be positive for M. chimaera, which indicates that there may be routes to contaminating the HCU other than that of a manufacturing point-source [
      • Svensson E.
      • Jensen E.T.
      • Rasmussen E.M.
      • Folkvardsen D.B.
      • Norman A.
      • Lillebaek T.
      Mycobacterium chimaera in heater–cooler units in Denmark related to isolates from the United States and United Kingdom.
      ]. Indeed, the water supplies within hospitals used to fill HCUs cannot be excluded as a source of M. chimaera contamination, and this reinforces the need for implementing a water safety plan and decontamination strategies by the water safety group [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ,
      • Department of Health
      ].

      Airborne transmission of M. chimaera from heater–cooler units to the surgical site

      Robinson et al. identified a genetic relatedness between 10 HCU M. chimaera isolates from four hospitals, but a patient M. chimaera isolate was not genetically related to the HCU M. chimaera isolates from that hospital, nor to the other HCU isolates, indicating that the HCUs were not the source of the infection in that patient [
      • Robinson J.O.
      • Coombs G.W.
      • Speers D.J.
      • Keehner T.
      • Keil A.D.
      • D'Abrera V.
      • et al.
      Mycobacterium chimaera colonisation of heater–cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing.
      ]. However, a high genetic similarity between M. chimaera isolates recovered from HCUs and patients in other studies is consistent with a role for HCUs in transmission [
      • Svensson E.
      • Jensen E.T.
      • Rasmussen E.M.
      • Folkvardsen D.B.
      • Norman A.
      • Lillebaek T.
      Mycobacterium chimaera in heater–cooler units in Denmark related to isolates from the United States and United Kingdom.
      ].
      Sax et al. hypothesized that contaminated water leaking from the circulation tubing or connectors may reach the turbulent airflow produced by fans in the lower body of the HCU [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ]. Gotting et al. also attributed the aerosolization of water droplets to the operating fans; specifically a fan located in the upper rear part of the HCU [
      • Götting T.
      • Klassen S.
      • Jonas D.
      • Benk Ch
      • Serr A.
      • Wagner D.
      • et al.
      Heater–cooler units: contamination of crucial devices in cardiothoracic surgery.
      ]. Public Health England, Porton Down, carried out a controlled laboratory study to investigate the microbial aerosols generated by a naturally contaminated (decommissioned) 3T HCU that had been in service since 2002 [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. A high-volume cyclone air sampler was used to confirm that the HCU released a bioaerosol.
      As was also demonstrated by Sax et al., this aerosol significantly increased with circulation of water and the highest level of aerosol was detected at the rear of the HCU [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ]. An aerodynamic particle sizer was used to carry out a more detailed examination of the HCU. A series of holes close to the flow and return pipes of both water circuits and a gap between the tank sealing plates were identified as areas of significant aerosol release (Figure 4). Flow visualization demonstrated the movement of smoke particles from these areas to the outside environment via the operation of a small cooling fan at the rear of the HCU (Figure 5 and Supplementary Video, Appendix A online). To date, these holes have only been observed in 3T HCUs manufactured before 2004 and may be due to the sealing plates shifting over years of operation. When the holes were sealed with mouldable putty the production of aerosols was greatly diminished [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ].
      Figure 4
      Figure 4Location of the gaps in the water tank top plates that contributed to the release of aerosols from the heater–cooler unit studied by Public Health England, Porton Down, UK.
      Figure 5
      Figure 5Use of a smoke tracer to visualize air flow path as it is extracted from the pumps/gaps associated with the water tanks and exiting the heater–cooler unit via the rear fan.
      The following is the supplementary data related to this article:
      Sommerstein et al. investigated the dispersal of M. chimera from a 3T HCU in a fully functional cardiac theatre fitted with an ultraclean air ventilation system validated to ISO EN 14644–3:2005. The vertical flow of laminar air from the ceiling should prevent aerosols from entering the operating field [
      • Sommerstein R.
      • Ruegg C.
      • Kohler P.
      • Bloemberg G.
      • Kuster S.P.
      • Sax H.
      Transmission of Mycobacterium chimaera from heater–cooler units during cardiac surgery despite an ultraclean air ventilation system.
      ]. The authors investigated aerosol transmission using a particle counter, a thermic anemometer, smoke dispersal and microbial monitoring when the HCU was switched off and when it was operating. When the HCU was operating and orientated such that its exhaust airflow (at the rear of the unit) was directed towards the surgical field, smoke from a source 20 cm from the HCU penetrated the ultraclean ventilation system and reached the operating field. This correlated with increased particle counts and the detection of M. chimera (settle plates) up to 5 m from the HCU. By combining these results with previous work using active air samplers, Sommerstein et al. concluded that the concentration of mycobacteria in the air was high enough to contaminate implant devices in the surgical field [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
      • Sommerstein R.
      • Ruegg C.
      • Kohler P.
      • Bloemberg G.
      • Kuster S.P.
      • Sax H.
      Transmission of Mycobacterium chimaera from heater–cooler units during cardiac surgery despite an ultraclean air ventilation system.
      ].

      Clinical features and diagnosis

      A number of investigations have demonstrated an increased risk of invasive M. chimaera infection in patients who have undergone cardiothoracic surgery, particularly valve replacement or repair [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Kohler P.
      • Kuster S.P.
      • Bloemberg G.
      • Schulthess B.
      • Frank M.
      • Tanner F.C.
      • et al.
      Healthcare-associated prosthetic heart valve, aortic vascular graft, and disseminated Mycobacterium chimaera infections subsequent to open heart surgery.
      ]. Although infections following coronary artery bypass grafts, vascular grafts, heart transplant, and left ventricular assist devices have been identified, risk assessment suggests that heart valve repair and replacement carry the highest risk [
      • Appenheimer A.B.
      • Diekema D.J.
      • Berriel-Cass D.
      • Crook T.
      • Daley C.L.
      • Dobbie D.
      • et al.
      Mycobacterium chimaera outbreak response: experience from four US healthcare systems.
      ]. A number of disseminated infections and secondary focal infections such as discitis have been reported [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Kohler P.
      • Kuster S.P.
      • Bloemberg G.
      • Schulthess B.
      • Frank M.
      • Tanner F.C.
      • et al.
      Healthcare-associated prosthetic heart valve, aortic vascular graft, and disseminated Mycobacterium chimaera infections subsequent to open heart surgery.
      ,
      • Appenheimer A.B.
      • Diekema D.J.
      • Berriel-Cass D.
      • Crook T.
      • Daley C.L.
      • Dobbie D.
      • et al.
      Mycobacterium chimaera outbreak response: experience from four US healthcare systems.
      ]. Patients affected have been primarily but not exclusively adults with a median age of 61 years (range: 36–76) which may reflect the age of those likely to require valve replacement surgery [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Kohler P.
      • Kuster S.P.
      • Bloemberg G.
      • Schulthess B.
      • Frank M.
      • Tanner F.C.
      • et al.
      Healthcare-associated prosthetic heart valve, aortic vascular graft, and disseminated Mycobacterium chimaera infections subsequent to open heart surgery.
      ]. The interval from surgery to diagnosis has ranged from three months to six years with a median interval of 17–18 months [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Appenheimer A.B.
      • Diekema D.J.
      • Berriel-Cass D.
      • Crook T.
      • Daley C.L.
      • Dobbie D.
      • et al.
      Mycobacterium chimaera outbreak response: experience from four US healthcare systems.
      ].
      Estimated risk of infection varies, in part a likely reflection of under-ascertainment of cases [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ]. Estimates from affected centres in Switzerland and the USA range from 1 per 100 to 1 per 1000 [
      • Sax H.
      • Bloemberg G.
      • Hasse B.
      • Sommerstein R.
      • Kohler P.
      • Achermann Y.
      • et al.
      Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery.
      ,
      • Centers for Disease Control and Prevention
      Contaminated devices putting open-heart surgery patients at risk.
      ,
      • Centers for Disease Control and Prevention
      CDC advises hospitals to alert patients at risk from contaminated heater cooler devices used during cardiac surgery.
      ]. In England, national risk assessment was possible through access to national health datasets. This identified an overall risk per patient undergoing heart valve surgery of 1 in 5000 between 2007 and 2015, although rising over time to reach 1 in 2000 in 2014 [
      • Public Health England
      Infections associated with heater cooler units used in cardiopulmonary bypass and ECMO. Information for healthcare providers in England Version 2.
      ].
      Patient outcome has been described as poor, which is probably multifactorial from late diagnosis and treatment, intrinsic drug resistance of M. chimera biofilm which may hinder the penetration, and action of antimicrobials in the cardiac and pulmonary tissue [
      • Kohler P.
      • Kuster S.P.
      • Bloemberg G.
      • Schulthess B.
      • Frank M.
      • Tanner F.C.
      • et al.
      Healthcare-associated prosthetic heart valve, aortic vascular graft, and disseminated Mycobacterium chimaera infections subsequent to open heart surgery.
      ,
      • Nagpal A.
      • Wentink J.E.
      • Berbari E.F.
      • Aronhalt K.C.
      • Wright A.J.
      • Krageschmidt D.A.
      • et al.
      A cluster of Mycobacterium wolinskyi surgical site infections at an academic medical center.
      ].
      Mycobacterium chimaera may also be misidentified, which could result in an underestimation of the number of cases. M. chimaera and M. intracellulare are genetically similar and most commercially available laboratory assays cannot differentiate M. chimaera from M. intracellulare. When 149 M. intracellulare isolates were reanalysed using 16S rRNA sequencing, 63% of them were found to be M. chimaera [
      • Soetaert K.
      • Vluggen C.
      • Andre E.
      • Vanhoof R.
      • Vanfleteren B.
      • Mathys V.
      Frequency of Mycobacterium chimaera among Belgian patients.
      ]. Reanalysis of M. intracellulare isolates from relevant sites in cardiothoracic patients may result in an increase in M. chimaera cases.

      How to prevent and control contamination of heater–cooler units

      Compatibility of manufacturer’s equipment

      To compound the issue of decontamination, hospitals using 3T HCUs with oxygenators manufactured by a different manufacturer (Maquet) have been informed that H2O2 should not be used as a disinfectant [
      • Maquet Getinge Group
      The use of hydrogen peroxide in heater–cooler systems with Maquet oxygenators. FSCA Number 2015-12-19.
      ]. Tests had shown that the diffusion rate of H2O2 through the Maquet oxygenator heat exchangers (polyurethane hollow fibres) may result in amounts of diffused H2O2 that exceed the daily allowable limits [
      • Maquet Getinge Group
      The use of hydrogen peroxide in heater–cooler systems with Maquet oxygenators. FSCA Number 2015-12-19.
      ]. Alternative chemicals recommended for the disinfection of HCUs by LivaNova include peracetic acid; however, a number of hospitals have concerns regarding the occupational exposure of staff to peracetic acid and are using clorina (active ingredient sodium p-toluenesulfonchloramide, recommended for Maquet oxygenators) for both the Maquet oxygenators and HCUs. However, hospitals should be aware that the advice from the regulator in the UK (Medicines and Healthcare products Regulatory Agency) is to follow the manufacturer’s instructions for cleaning and disinfecting these devices and the associated components at all times, as the impact on the safety and performance of the devices are unknown and such activities may invalidate the manufacturers’ warranty [
      • Medicines and Healthcare products Regulatory Agency
      Heater–cooler devices used in cardiac surgery – risk of infection with Mycobacterium species update.
      ].

      Decontamination recommendations

      To date, all M. chimaera infections have been attributed to the 3T HCUs. However, microbial contamination of HCUs is widespread and is not specific to particular manufacturers or models [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Svensson E.
      • Jensen E.T.
      • Rasmussen E.M.
      • Folkvardsen D.B.
      • Norman A.
      • Lillebaek T.
      Mycobacterium chimaera in heater–cooler units in Denmark related to isolates from the United States and United Kingdom.
      ]. Such contamination implies a systemic failure of the decontamination regimen resulting in the formation of biofilms. The environmental conditions and usage of the HCU favours the growth of a wide range of micro-organisms [
      • Chand M.
      • Lamagni T.
      • Kranzer K.
      • Hedge J.
      • Moore G.
      • Parks S.
      • et al.
      Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients.
      ,
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ,
      • Haller S.
      • Holler C.
      • Jacobshagen A.
      • Hamouda O.
      • Abu Sin M.
      • Monnet D.L.
      • et al.
      Contamination during production of heater–cooler units by Mycobacterium chimaera potential cause for invasive cardiovascular infections: results of an outbreak investigation in Germany, April 2015 to February 2016.
      ]. All manufacturers provide advice for sampling and decontamination of HCUs on their websites.
      In terms of the 3T HCU, in 2015 LivaNova advised the following: decant HCUs of water every two weeks, refill with filtered (0.2 μm) tap water and add 100 mL of medical grade 3% hydrogen peroxide (H2O2) to the HCU tanks. Fifty millilitres of 3% H2O2 should then be added every five days. A full system HCU decontamination using Maranon (>30% sodium hypochlorite) was advised every three months [
      • Anonymous Sorin Group
      Field Safety Notice, Cardiac Surgery. Mycobacterium risks disinfection and decontamination of Sorin heater cooler devices.
      ]. There was also a change in the cleaning and disinfection guidance in 2015 to require more frequent disinfection of the water circuit (i.e. disinfection every two weeks rather than every three months) with specified disinfectant solutions including peracetic acid, weekly water changes, and the addition of hydrogen peroxide solution to the water to act as a preservative and to further prevent biofilm formation [
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ,
      • Schreiber P.W.
      • Kuster S.P.
      • Hasse B.
      • Bayard C.
      • Rüegg C.
      • Kohler P.
      • et al.
      Reemergence of Mycobacterium chimaera in heater–cooler units despite intensified cleaning and disinfection protocol.
      ]. In terms of microbiological sampling LivaNova advised users to sample the water prior to disinfection for heterotrophic bacteria, coliforms, P. aeruginosa and NTM. During usage in the operating theatre, users were also advised to carry out biweekly microbiological water and air sampling.

      Efficacy of decontamination assessed by microbiological testing

      Studies have indicated that the decontamination procedures provided by the manufacturer do not sufficiently inhibit the growth of micro-organisms in HCUs [
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ,
      • Schreiber P.W.
      • Kuster S.P.
      • Hasse B.
      • Bayard C.
      • Rüegg C.
      • Kohler P.
      • et al.
      Reemergence of Mycobacterium chimaera in heater–cooler units despite intensified cleaning and disinfection protocol.
      ]. Garvey et al. found that, using the initial decontamination advice provided by the manufacturer, environmental Gram-negative bacteria, Pseudomonas spp., fungi and M. chimaera were recoverable from the HCU [
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ]. Working with the manufacturers, Garvey et al. carried out two consecutive full-system HCU decontamination cycles with Maranon (420 mL, >30% sodium hypochlorite). In addition the HCUs were subjected to weekly water changes (with filtered tap water), daily addition of medical grade 3% H2O2 (100 mL) and a weekly full-system decontamination using Maranon. However, based on the manufacturer’s guidance, water samples from the HCU remained microbiologically unacceptable [
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ].
      It was only after complete refurbishment and replacement of the internal tubing by the manufacturer, followed by the application of the disinfection regimen, that the weekly HCU microbiological sampling counts were reduced to a satisfactory level [
      • Gukop P.
      • Tiezzi A.
      • Mattam K.
      • Sarsam M.
      Emergency management of heat exchanger leak on cardiopulmonary bypass with hypothermia.
      ]. The addition of an effective disinfectant to an HCU with minimal biofilm (either because it is new or refurbished) would be expected to control the survival and growth of water-borne micro-organisms, which may explain the lack of detection of M. chimaera from HCUs after refurbishment by LivaNova [
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ]. In addition, the daily water changes and subsequent dilution of slow-growing NTM may have resulted in the bacterial numbers being below the level of detection in water and air sampling [
      • Garvey M.I.
      • Ashford R.
      • Bradley C.W.
      • Bradley C.R.
      • Martin T.A.
      • Walker J.
      • et al.
      Decontamination of heater–cooler units associated with contamination by atypical mycobacteria.
      ]. Replacement of the internal tubing and renovation of the HCUs is a service offered by the manufacturer as a way of eradicating M. chimaera from the units. However, this service is only available for units that were manufactured after 2008 and there is an associated cost. More recently a number of perfusionists in the UK have been trained by the manufacturer to undertake this refurbishment (J. Walker, personal communication).
      Whereas enhanced disinfection may fail to eradicate NTM in water or biofilm phase, Schreiber et al. found that aerosolized M. chimaera were not detectable in brand new HCUs that were undergoing the manufacturer’s recommended enhanced disinfection regime, suggesting that the infection risk via this route may have been reduced [
      • Billings N.
      • Birjiniuk A.
      • Samad T.S.
      • Doyle P.S.
      • Ribbeck K.
      Material properties of biofilms – a review of methods for understanding permeability and mechanics.
      ]. To date there has been no detailed investigation of risk to patients in terms of the number of M. chimaera in the water phase compared to the number present in aerosols, which would have assisted in the establishment of a risk assessment based on quantifiable results.
      It is clear that the increased disinfection and sampling regimens recommended by the manufacturer have added a considerable burden of duties and costs for perfusionists, cardiology and theatre staff. The microbiological analysis, air sampling, air filters and disinfectants have been estimated at £34,391.54 per year for one UK hospital operating eight LivaNova 3T HCUs (Table I) (J. Walker, personal communication). In addition, conversion of an existing space to be used for decontamination has also been estimated by that hospital to cost an additional £28,444.
      Table IEstimated costs for one hospital to undertake testing of eight LivaNova 3T heater–cooler units
      Microbiological analysis/air samplingEstimated costAnnual costs
      Legionella (water)£18 × 8 samples (every 3 months)£576.00
      Pseudomonas aeruginosa (water)£7.65 × 8 samples (every month)£734.00
      Environmental mycobacteria (water)£50.40 × 8 samples (every month)£4,838.00
      Total air analysis costs for 8 machines£150 × 8 machines (every month)£14,400.00
      Water filtersBox of 2 for 2 outlets per year£903.00
      Peracetic acidFor 8 units throughout one year£1,326.00
      Staff timeFor 8 units throughout one year£11,614.50
      Total£34,391.50 (excl. VAT)
      Whereas disinfection has not completely removed the presence of M. chimaera in the water tanks, there have so far been no new cases of M. chimaera infections identified originating from surgery since the introduction of the enhanced decontamination strategies. However, it may still be too early to speculate as we remain well within the recognized incubation period for the infection. Indeed other NTM strains, notably M. abscessus linked to hospital tap water, have also been associated with cardiac surgery patient infections [
      • Baker A.W.
      • Lewis S.S.
      • Alexander B.D.
      • Chen L.F.
      • Wallace Jr., R.J.
      • Brown-Elliott B.A.
      • et al.
      Two-phase hospital-associated outbreak of Mycobacterium abscessus: investigation and mitigation.
      ].

      Heater–cooler unit containment and relocation strategies

      Reliable, effective decontamination of HCUs and maintaining the microbiological quality of the water is difficult and many hospitals have adopted alternative control strategies. In line with guidance, hospitals have been positioning the HCU as far from the patient area as is feasible, as well as directing the rear fan away from the operating field and, where possible, close to an air extraction port in the theatre [
      • Public Health England
      Infections associated with heater cooler units used in cardiopulmonary bypass and ECMO. Information for healthcare providers in England.
      ]. An alternative approach involves enclosing the water reservoirs and tubing of the HCU within an air-tight ‘biosafety box’ and extracting the air from within the box to the outside of the theatre [
      • Sommerstein R.
      • Jenni H.
      • Carrel T.
      • Marschall J.
      Cardiac surgery, nosocomial infection, and the built environment.
      ]. Another is to place the HCU outside of the operating theatre. Gotting et al. positioned the HCU through a doorway; however, the positioning of the tubing connecting the cardiopulmonary bypass machine to the HCU prevented the theatre door from closing [
      • Götting T.
      • Klassen S.
      • Jonas D.
      • Benk Ch
      • Serr A.
      • Wagner D.
      • et al.
      Heater–cooler units: contamination of crucial devices in cardiothoracic surgery.
      ]. This may have been the reason why the air samples from the operating room were positive and underlines the importance of controlling air flow in the whole operating area.
      In Bern University Hospital, four new cardiac operating theatres have been built with ante-rooms specifically designed for housing the HCUs in 2009 prior to the current M. chimaera outbreaks [
      • Sommerstein R.
      • Jenni H.
      • Carrel T.
      • Marschall J.
      Cardiac surgery, nosocomial infection, and the built environment.
      ]. The tubing passes into each operating room via cavities in the walls, removing the risk of exhaust air from the HCU entering the operating room [
      • Sommerstein R.
      • Jenni H.
      • Carrel T.
      • Marschall J.
      Cardiac surgery, nosocomial infection, and the built environment.
      ]. A UK hospital has recently taken a similar approach by building modular plates into the walls that seal the tubing passing through the wall (Figure 6, Figure 7). The costs of this intervention, including revalidation of the theatre air flows, has been estimated at £15,000. Figure 6, Figure 7 also show the newly recommended polyethylene tubing (blue) for use with the HCUs to try to reduce biofilm formation. Perfusionists’ comments are that the tubing is less flexible, and, being opaque, the perfusionists are unable to see whether there has been a blood/water breach in the system (J. Walker, personal communication).
      Figure 6
      Figure 6Positioning of the heater–cooler unit outside the operating theatre with tubing passing through modular plates in the wall.
      Figure 7
      Figure 7Position of the tubing through the wall into the operating theatre, using anti-slip matt housing and connection to the cardiopulmonary bypass equipment.
      Assessment and revalidation of the operating theatre airflows is essential to ensure that they are not disrupted by any changes. The use of longer lengths of tubing may cause further complications such as the requirement for larger volumes of water, inefficient heat transfer and the inability to maintain temperature control with the additional possibility of increased trips and falls. Any containment and relocation strategy should involve the manufacturer.
      In due course the manufacturers may be able to (i) implement an engineering solution to upgrade existing devices to prevent dissemination of aerosols from the HCUs and/or (ii) design new HCUs to prevent the growth of NTM in the water and to ensure that aerosolization is minimized during the operation of the HCU. However, these units should be fully tested and validated for microbial control and aerosol containment by independent and objective scientists.

      Implications for other thermoregulatory equipment

      The concern surrounding post-surgical M. chimaera infections had until recently been limited to HCUs. However, HCUs are also used for extracorporeal membrane oxygenation (ECMO) and similar thermoregulatory devices are required for circulating-water mattress to maintain body temperature [
      • Trudzinski F.
      • Schlotthauer U.
      • Kamp A.
      • Hennemann K.
      • Muellenbach R.M.
      • Reischl U.
      • et al.
      Clinical implications of Mycobacterium chimaera detection in thermoregulatory devices used for extracorporeal membrane oxygenation (ECMO), Germany, 2015 to 2016.
      ,
      • Kim H.Y.
      • Lee K.C.
      • Lee M.J.
      • Kim M.N.
      • Kim J.S.
      • Lee W.S.
      • Lee J.H.
      Comparison of the efficacy of a forced-air warming system and circulating-water mattress on core temperature and post-anesthesia shivering in elderly patients undergoing total knee arthroplasty under spinal anesthesia.
      ]. M. chimaera was detected in nine out of 18 water samples taken from 10 different HCUs used for ECMO devices [
      • Trudzinski F.
      • Schlotthauer U.
      • Kamp A.
      • Hennemann K.
      • Muellenbach R.M.
      • Reischl U.
      • et al.
      Clinical implications of Mycobacterium chimaera detection in thermoregulatory devices used for extracorporeal membrane oxygenation (ECMO), Germany, 2015 to 2016.
      ]. Of 76 ECMO patients who had bronchial specimens analysed for mycobacteria, M. chimaera was found in three individuals without signs of mycobacterial infection at the time of sampling. It would be prudent for water safety groups to ensure that HCUs used in ECMO are assessed as part of the water safety plans to ensure that the risk is reduced for those who may be exposed via this route.

      Conclusion

      There is an ongoing international outbreak of M. chimaera infection following cardiac surgery. Although a transmission risk has been identified, it is essential to recognize that many cardiac procedures, which can be life-saving, are dependent on these devices. The low risk of infection must be balanced with usually much higher risks from delaying surgery.
      The genetic similarity between case (patient) and environmental isolates supports the hypothesis that M. chimaera is transmitted to the surgical site via the aerosolization of contaminated water from within HCUs. The water tanks of HCUs have been shown to contain a wide variety of micro-organisms including extensive biofilms on a number of different components which likely contributes to the survival of M. chimaera.
      The evidence from whole genome sequencing of M. chimaera in the UK , USA, Australia/New Zealand and Europe has demonstrated close relatedness between patient and device isolates, supporting the hypothesis of transmission from the HCUs and suggestive of a point-source outbreak from the manufacturer’s factory; within a wider potential risk of microbial transmission from this type of device. Rigorous disinfection regimens have been recommended by the different manufacturers, but these protocols and microbiological tests can be time-consuming and resource-intensive for hospitals, in terms of personnel and cost.
      Whereas disinfection has not completely removed the presence of M. chimaera in the water tanks, there have so far been no new cases of M. chimaera infections identified originating from surgery after the introduction of the enhanced decontamination strategies. However, it may still be too early to speculate and indeed other NTM strains, notably M. abscessus linked to hospital tap water, have been associated with cardiac surgery patient infections. Therefore, in order to reduce the risk of future infection, it is critical that hospitals are aware of the risk and follow strict decontamination procedures of HCUs. Water safety groups should work with users to ensure control of microbiological risks from HCUs, and that water system management is embedded in routine infection control practice.
      It is not clear why M. chimaera is currently the dominant clinical infection, when other species of NTM are found regularly within the HCUs. It may be an artefact of methods used for surveillance; it may relate to M. chimaera being more widespread rather than being only from a point source; or perhaps it may relate to an as yet undescribed biological characteristic of the species facilitating survival either in the HCU or in the patient.

      Acknowledgements

      The opinions expressed in this manuscript are those of the authors and not necessarily those of Public Health England or any other Government agency. The authors would like to acknowledge the input of all those who were represented on the incident management team and to Papworth Hospital Trust for their financial data on the costs incurred in the management of their heater–coolers.

      Conflict of interest statement

      None declared.

      Funding sources

      None.

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